The purpose of this work is to provide information on the mode of action of nerve growth factor. Nerve growth factor is required for the development and maintenance of the sympathetic and sensory nervous systems and may be a prototype of the many factors now known to be involved in the growth and maturation of various cell types. The mechanism of action is of interest because it should lead to an understanding of the way in which gene expression is regulated in neurons and how such factors induce the synthesis of specific enzymes for the biosynteshis of neurotransmitters or for the morphological changes necessary for the neron to communicate with other cells through its ability to form synapses. In turn, such information may lead to an understanding of the development and differentiation of the non-dividing, non-replaceable neurons of the central nervous system and to information about the tumors which arise from them.